Underwater pipelines for oil, gas and other fluids require maintenance to keep their interior passageways clear, and for this purpose a robotic internal pipeline scraper is often used. Such a scraper is inserted at one entrance to the pipeline and is propelled to pass along the pipeline to an exit, scraping the interior sides of the passageway free from buildup as it goes.
Necessarily, in order to scrape the sides, the scraper must contact the sides with an appropriate degree of force. This operation creates a problem when the pipeline contains a branch connection, as most pipelines do. The branch connection may be in the form of a Y or T connection, but in any case the connection fitting makes an opening in the main passageway. A scraper making its way along the main passageway may turn into the opening at the branch connection and become jammed.
FIG. 1 illustrates this problem in the context of a typical pipeline scraping operation. As shown therein, a 30 inch pipeline 10 has a 24 inch tie-in T-section 12 connected therein with a corresponding opening 14 to permit passage of the conveyed liquid. An auxiliary pipeline 16 is connected to the T-section 12 by a conventional bolted flange 18. The direction of flow of the conveyed liquid is indicated by the arrow A inside the pipeline 10.
In FIG. 1, a pipeline scraper 20 has been introduced into the main pipeline 10 from the upstream direction indicated by arrow B at the left of the figure and is traveling downstream, toward the right of the figure. At a first position 22 within the pipeline 10, the scraper 20 is in contact with all interior sides of the pipeline 10 with sufficient force to be scraping those sides free of accumulated debris. However, when the scraper 20 reaches position 24, it encounters the opening 14 and may tilt into it, with the lead end 26 of the scraper 20 entering the T-section 12 and becoming jammed against the far corner 28. As a result, the pipeline 10 goes out of service and must be repaired.
The only way to repair the pipeline 10 under these conditions is to flush the pipeline 10 and then send one or more divers underwater to locate the jammed connection and lift the pipeline 10 itself onto a work barge, where the jammed scraper 20 can be removed by cutting out the section containing the fouled fitting and putting an appropriately configured new branch fitting in its place. The pipeline 10 can then be reclosed and deposited back under the sea. Needless to say, this is a very expensive operation and has considerable associated environmental risks.
Other robotic devices, such as monitoring devices, are also used to traverse the pipelines and may similarly become jammed at the branch openings.
The use of scrapers and such other devices is a relatively new development as compared with the age of certain existing pipelines that are still in service. Their use can enhance and extend the useful life of older pipelines, but this was not contemplated when the older pipelines were put in place.
Now that the use of scrapers has become more common, pipeline fittings are provided with the necessary guards or longitudinally extending bars that will preclude the entry of the scraper into the opening into the branch line. It would be highly desirable to retrofit the existing pipelines with such guard bars over the branch openings, but the only way currently known for doing so involves the pipeline lifting operation described above, making it prohibitively expensive to engage in any general overhaul.
Moreover, in many cases trunk lines cross under or over other trunk lines, flow lines, power lines and/or communication cables, so that individual trunk lines often cannot be lifted to the surface without wholesale disruption of the entire pipeline system.
Consequently, while retrofitting is highly advantageous, it is possible in accordance with conventional techniques only in limited circumstances and only at great expense.